Traditionally, ultrasound has been applied through hand-held transducer probes/heads in diagnostic and therapeutic scenarios. The ‘head’ has to be continuously moved for both practical and safety reasons. In diagnostic scanning, dynamic images are displayed on a screen and movement allows the object to be viewed from multiple angles. The time averaged intensities are lower in diagnostic ultrasound but nevertheless constant movement also minimises cumulative ultrasound exposure of a given volume of soft or hard tissue. Therapeutic ultrasound (e.g. physiotherapy) utilises higher (averaged) intensities of ultrasound and is employed purely to provide a physiological response, e.g. muscle repair following a sporting injury.
Constant movement of hand-held devices is important to avoid over and under exposure. Over-exposure can lead to over-heating/thermal damage and also standing waves being created with the potential to cause lysis of cells. Conversely, under-exposure will reduce the amount of ultrasonic energy received by a particular area of the body and therefore cause reduced therapeutic benefit.
Relying on manual movement of the device is unreliable and cannot guarantee even coverage and therefore even exposure. Some areas will not receive the same level of treatment as others and are highly dependent on the abilities of the practitioner to keep the device moving at a constant steady speed potentially over a 20-30 minute period leading to arm/wrist/hand fatigue and uneven treatment of the patient. Electronic movement over an array of transducers will obviate operator error normally associated with uneven/erratic movement of an otherwise hand-held device.
The underlying technology on which this invention is based is thin, flexible patches or bandages containing an array of ultrasound transducers that operate in close contact with complex bodily surfaces such as the face. As with all applications and geometries of applied ultrasound, to perform correctly there needs to be an ‘air-free’ acoustic path for the ultrasound from transducer surface to skin surface. Air cavities/bubbles etc would severely impede propagation of ultrasound due to their significantly lower acoustic impedance causing reflection and refraction of the propagating wave so lowering the intensity of ultrasound impinging on and propagating through the skin.
Such a flexible ultrasound patch would thus need to conform closely to bodily surfaces and avoid, as much as possible, any buckling of the patch to allow air spaces to come between the patch and the skin. This problem may be overcome somewhat by using free-flowing gels that fill any air-spaces.
Arrays of transducers need to be wired to enable every element in that array to be activated. This wired array also needs to be encapsulated to prevent water (e.g. coupling gel) ingress and general soiling. Encapsulating materials that have some inherent elasticity may allow moulding to doubly curved surfaces, but the associated electrical circuit contained within is most unlikely to allow such complex bending.
Within the ultrasonic patch, each of the transducers require robust electrical interconnection that can withstand frequent and numerous flexing/bending. Failure of the connections could result in a transducer failing to operate and may even cause failure of entire sub-groups of transducers.
Therefore, there needs to be an electrical interconnecting system that can withstand repetitive bending while allowing moulding to complex surfaces.
Materials applied to the surface of the human body or other complex shapes typically employ some degree of tension in many directions to keep the material in contact with that object, e.g. Lycra/Spandex™ clothing. Flexible sheets of material such as paper can easily conform to singly curved shapes, e.g. cylindrical, but have difficulty in conforming to doubly curved shapes, e.g. a sphere.
It is known to mount an array of transducers on a flexible printed circuit board (flexi-PCB). Previous studies (e.g. Arunachalam et al., 2008, ‘Performance evaluation of a conformal TMS sensor array’ Int. J. Hyperthermia, 24(4), 313-325) describe flexible PCB mounted temperature sensors for measuring skin surface temperatures. The study employed multi-layer Kapton® polyimide film which is known to have stable mechanical, physical and thermal properties as well as high tensile strength and folding endurance (285 k cycles) suitable for use when wrapped around the human torso.
However, the Arunachalam et al. study uses a single continuous sheet of flexible PCB. That sheet of flexi-PCB would curve and bend to conform to a cylindrical geometry, but not to a doubly curved surface such as a sphere or a saddle point.
WO 2008137030(A1), entitled ‘A flexible conformal ultrasonic imaging transducer and system’, discloses a system that is intended for, but not limited to, ultrasonic imaging via send and receive ultrasonic pulses. A conformal flexible transducer array for contact to various parts of the human body is disclosed. However, like the conformal TMS array described in Arunachalam et al, the transducers are mounted on a continuous sheet of Kapton® polyimide flexible printed circuit substrate which would limit the number of bending directions to one, hence only achieving close conformity to a singly curved surface such a cylinder.
U.S. Pat. No. 5,735,282 (A) discloses the mounting of a linear 1D array of ultrasound transducers on multiple flex circuit segments, wherein sub-groups of the transducers in the linear array are respectively mounted on different flex circuits. It is stated that in an array of 128 PZT elements, each on a 0.3 millimeter pitch, it may be advisable to have eight or more individual flex segments. The more flex segments that are used, the greater the ability of the array to flex. However, when multiple layers of flex circuit are folded on top of one another, there can be a disadvantage associated with the increased overall thickness of the assembly and the increased vulnerability to cross-talk. In fact, the linear array must only flex along the azimuth, although because of the way the arrangement is constructed, the individual flex circuits must also simultaneously fold around the back of the array.